Nickel-iron magnetic alloys comprising chromium and molybdenum

ABSTRACT

Magnetic alloys comprised of iron, nickel, chromium and molybdenum in which a weight ratio of nickel to iron is selected in a range of about 0.54 to 1.5.

United States Patent [191 Kuroda [451 Oct. 29, 1974 NICKEL-IRON MAGNETICALLOYS COMPRISING CHROMIUM AND MOLYBDENUM Nobukazu Kuroda, Kanagawa-ken,Japan Inventor:

Assignee: Sony Corporation, Tokyo, Japan Filed: Jan. 19, 1973 Appl. No.:325,144

Foreign Application Priority Data Jan. 27, 1972 Japan 47-010146References Cited UN1TED STATES PATENTS Smith et al 75/170 Howe 75/170Parkin 75/170 Muzyka et al. 75/170 Onyshkevych.... 148/3155 Holtz et al.75/170 Primary Examiner-Walter R. Satterfield Attorney, Agent, orFirm-I-lill, Gross, Simpson, Van Santen, Steadman, Chiara & SimpsonABSTRACT Magnetic alloys comprised of iron, nickel, chromium andmolybdenum in which a weight ratio of nickel to iron is selected in arange of about 0.54 to 1.5.

4 Claims, 15 Drawing Figures minimum agamew NICKEL-IRON MAGNETIC ALLOYSCOMPRISING CHROMIUM AND MOLYBDENUM .BACKGROUNDOF THE'INVENTION l Fieldof the Invention The present invention relatesgenerally to magneticalloys and more particularly to magnetic alloys which are useful asmagnetic shield material for magnetic heads.

v2. Description of the Prior Art Generally, magnetic shield materialsfor a magnetic head have been formed of an iron-nickel binary alloyconsisting of 79 weight percent nickel and 21 weight ,percentliron, thatis, a-so-called 79-Permalloy. The 79- Permalloy has magneticcharacteristics whereby its initial permeability is more than 3,000 andits magnetic flux density is more than 3,-O0Gausses. Such magneticcharacteristics are most preferable for a magnetic shield material thatis to be used in combination with a magnetic head. Such a 79-Permalloy,however, has the drawback that its manufacturing cost is rather high dueto the fact that it is difficult to work during manufacturing processesand that it contains a large amount of nickel which is uneconomical.

45-Permalloy (containing 45 weight percent nickel and 55 weight percentiron) has been also proposed, which is relatively easily processedduring manufacturing due to the fact that it contains less nickel buthas enough magnetic characteristics to function as a magnetic shieldmaterial for a magnetic head. However, rust is apt to be formed on such45-Permalloy. In other words, rust is gathered on a shield case or platemade of 45-Permalloy during a rinsing step in a typical manufacturingprocess or after manufacturing, so that 45- Permalloy is not practicallyuseful.

Further, both of the 79-Permalloy and 45-Permalloy are about 120-130 inhardness (Vickers scale) and show poor wear resistance. For this reason,a magnetic shield and/or head composed of such Permalloys has a shortlife time and is undesirable for use in magnetic recording and/orreproducing on and/or with a magnetic tape using chromium dioxidepowders.

SUMMARY OF THE INVENTION The present invention provides a magneticshield material wherein a part of the iron or the nickel content in aniron-nickel alloy is substituted by chromium and molybdenum to providean iron-nickelchromium-molybdenum quadruple or quaternary alloy, bywhich the drawbacks encountered in the prior art are avoided.

Accordingly, it is an object of the present invention to providemagnetic alloys which are easily rolled or worked and manufactured atlow cost.

It is a further object of the present invention to provide a magneticalloy which is high in wear resistance and suitable as a magnetic shieldmaterial for a magnetic head.

It is a yet further object of the present invention to provide ananti-corrosive or anti-rust magnetic alloy which is manufactured at lowcost.

The other objects, features and advantages of the present invention willbe apparent from the following description taken in conjunction with theaccompanying drawings.

LII

BRIEF DESCRIPTION OF THE DRAWINGS thus obtained alloy;

FIG. 3 is a graph showing relationship between amounts of 'Cr, Mo and Niadded to ternary alloys of (FeNi)Cr, (FeNi)Mo and a binary alloy(-FeNi)Ni, respectively, and the rust forming rates of thus manufacturedalloys;

FIG. 4 is a graph sowing relationship between the ratio of Ni/Fe in aternary alloy (FeNi)Cr and the amount of Cr therein at a-critical pointwhere the rust disappears during rust forming test in salt water;

FIG. 5 is a graph showing a characteristic curve which illustrates thestate of magnetic flux modification produced by changing the amount ofNi in an FeNi aly;

FIG. 6 is. a graph showing magnetic characteristic curves, such as theinitial permeability 110, maximum magnetic flux density B, and coerciveforce He, relative to the amount of Cr in a ternary alloy (Fe Ni ,Cr,;

FIG. 7 is a graph showing magnetic characteristic curves, such as theinitial permeability uo, maximum magnetic flux density B, and coerciveforce Hc relative to the amount of M0 in a ternary alloy (Fe Ni 1;

FIG. 8 is a composition diagram showing rust forming rate of quadrupleor quaternary alloy (FeNi)CrMo relative to the amounts of Cr and Motherein;

FIG. 9 is a composition diagram showing the magnetic permeability p0,coercive force Hc, and magnetic flux density B of a quadruple alloy(FeNi)CrMo relative to the amounts of Cr and Mo therein;

FIG. 10 is a graph showing magnetic characteristics, such as themagnetic permeability no, coercive force Hc, and magnetic flux density Bof a quadruple alloy (Fe Ni Cr Mo relative to the amount of Mo therein;

FIG. 11 is a perspective view of a magnetic head using the magneticalloys according to the present invention; and

FIGS. 12A to 12D inclusively, are cross-sectional views, respectively,illustrating surface conditions of the magnetic head depicted in FIG. 11after a wear test thereof wherein the shield case and magnetic core ofthe magnetic head are changed in composition.

DESCRIPTION OF THE PREFERRED EMBODIMENTS It has been discovered thatwhen chromium (Cr), which improves anti-corrosive characteristics ofmagnetic material, and molybdenum (Mo), which enhances permeability ofmagnetic material, are added to a composition consisting mainly of iron(Fe) and nickel (Ni) in small but definite amounts, a magnetic alloyhigh in permeability can be obtained even if the amount of nickel isreduced therein.

A magnetic alloy according to the present invention comprises amagnetically heat-treated alloy composed of about 6 to 12 weight percentof chromium (Cr),

about 0.5 to 8 weight percent of molybdenum (Mo), and the balance ofiron (Fe) and nickel (Ni) in which the ratio of nickel (Ni) to iron (Fe)is selected from about 0.54 to about 1.5. The magnetic alloy of theinvention has the necessary magnetic flux density and initialpermeability required for a magnetic shield material for a magnetichead.

The magnetic alloys of the invention will be illustrated by variousexemplary embodiments set forth hereinafter and their characteristicswill be compared to certain other compositions so that those skilled inthe art may undertake various alterations and modifications to arrive ata particularly desired magnetic alloy composition.

FIGS. 1 to inclusive, are, respectively, graphs or diagrams which showthe results of experiments carried out for determining the preferredcompositions of magnetic alloys in accordance with the principles of theinvention.

FIGS. 1 and 2, respectively, show the relationship between the amountsof chromium (Cr) and molybdenum (Mo) added to an iron-nickel (FeNi)alloy and initial permeabilities of the alloys thus obtained.

FIG. 1 is a graph showing the relationship between the amount of Cr (byweight percent added to an FeNi alloy and the initial permeability no ofa ternary alloy FeNiCr thus obtained. In the graph of FIG. I, theabscissa represents the amount of Cr in weight percent and the ordinaterepresents the initial permeability, #0. In this example, the amount ofFe was selected to be substantially equal to that of Ni in weightpercent. From the characteristic curve, namely the relationship shown inFIG. 1, it will be observed that as the amount of Cr added to the FeNialloy increases, the initial permeability to of FeNiCr alloy decreases.

FIG. 2 is a graph showing the relationship between the amount of Moadded to an FeNi alloy in which the amount of Fe was selectedsubstantially equal to that of Ni in weight percent and the initialpermeability, no of the resultant FeNiMo alloy. It will be observed fromthe characteristic curve, namely the relationship depicted in FIG. 2,that even if M0 is added to the FeNi alloy in relatively small amounts,the FeNiMo alloy has an initial permeability, #0 higher than that of theFeNiCr alloy by about a factor of 10.

FIG. 3 is a graph for showing the relationship between the rust formingrates in the alloys represented by (FeNi)Cr at curve I, (FeNi)Mo atcurve II and (FeNi)Ni at curve III with various amounts of Cr, Mo and Niwhen the alloys were heated at 600 C. in air. In this graph, theabscissa represents the amounts of Cr, Mo, and Ni, respectively, whilethe ordinate shows the rust forming rates. In this example, the amountsof Fe and Ni were substantially equal in each case. In the graph of FIG.3, the dotted-line curve IV represents the similar relationship of thebinary alloy Fe Ni Marked solid points on these curves show thecomposition where rust was formed on the alloys, which were immersed insalt water of 8 percent and 1.5 percent; marked points having anasterisk show the compositions where rust was formed on the alloys,which were immersed in salt water of 8 percent in 200 hours; and markedcircles show the compositions where no rust was formed on the alloys,which were immersed in salt water mentioned above. The rust forming rateon the x period during which the alloys were heated at 600 C. in air,while the abscissa represents the adding rate of the alloy materials,namely, Cr, Mo and Ni by weight percent. As is apparent from FIG. 3, analloy having high anticorrosive characteristics is attained when variousamounts of Cr are added thereto.

FIG. 4 is a graph showing a characteristic curve a between a ratio of Nito Fe in weight percent, namely, the ratio Ni,,/Fe, and the criticalamount x (in weight per cent) of Cr, where no rust is formed on thealloy of such a composition as represented by the formula (Fe1 ,,Ni,,)1-Cr which is immersed in salt water. In this case, the critical amountwhere no rust is formed is taken as the rust forming rate K of the79-Permalloy, which has been employed widely as material for a magnetichead and is known not to present a problem of forming rust in practicaluse. This standardized rust forming rate K is equal to 0.01. It is, ofcourse, desirable that a select material of the invention have a rustforming rate K smaller than 0.0I. In FIG. 4, the ordinate represents theamount of Cr in weight percent and the abscissa ratio of Ni /Fe inweight percent. The scale 0.54 on the abscissa shows the composition ofFe Ni s, the scale 1.0 the composition of 1 5mm,, and the scale 1.5 thecomposition of FemNiao. respectively.

In the graph of FIG. 4, the area I) under the charac' teristic curve ashows an area where rust tends to form on the alloys represented by thecompositions in this area. Rust hardly forms on alloys represented bythe compositions in the area (2) above the characteristic curve a.

As will be noted from FIG. 4, in compositions where the value of theratio Ni /Fe is greater than 1.5, that is to say, the amount of Ni tothat of Fe is greater than weight percent, no rust is formed on thealloy, unless an amount of Cr is added thereto. Accordingly, with thepresent invention the rust characteristic is improved by reducing theamount of Ni, which is uneconomical and by adding an amount of Cr.Further, since Mo contributes less to the improvement of the rustrepelling characteristic of the alloy, the ratio of Ni/Fe in the binaryalloy FeNi is selected smaller than 1.5, for example, as in Fe Ni On theother hand, the alloy of FeNi is mainly divided into two phases, one ofwhich is in the phase of iron Fe containing Ni from 0 to 30 weightpercent, as seen from FIG. 4 above, and the other phase is in the phaseof nickel Ni containing Ni from 40 to I00 weight percent. In the formerphase (within the range containing Ni from 0 to 30 weight percent), thealloy has the crystal structure of body center cubic lattice at lowtemperature but at high temperature it is transformed into a crystalstructure of face center cubic lattice, while in the latter phase(within the range containing Ni from 40 to I00 weight percent), thealloy becomes a solid solution of face center cubic lattice irrespectiveof temperature. On the boundary (Ni being 30-40 weight percent) betweensuch two phases, the saturation magnetization of the alloy is loweredslightly and at the same time its Curie point is greatly lowered, sothat its magnetic flux density B is also greatly lowered at roomtemperature. Accordingly, the alloy becomes substantially non-magnetic,and this characteristic is illustrated in FIG. 5. Even when attempts toincrease the magnetic flux density B from the point mentioned above aretried, by adding Mo thereto, at least the alloy FeNi must contain morethan 35 weight percent of Ni and hence the ratio of Ni/Fe in weightpercent must be at least 0.54, that is to'say, Fe Ni or more as shown atFIG. 4.

FIG. 6 is a graph showing the relationship betweentheinitialpermeability yo, magneticflux density B, and coercive forceHe, of a ternary alloy (FeNi), Cr, .(along the ordinate) and where theamount x of Cr con- :tained therein is changed and the alloy FeNi isselected to be Fe Ni which is the ratio of Ni/Fe slightly .greater thanthat of the FeNi alloy which has a greatly decreased magnetic fluxdensity B, as described just above vin conjunction with FIG. v5. In thegraph.of FIG. 6, reference numeral I represents the curve of the initialpermeability uoynumeral Il represents the curve of the magnetic fluxdensity B, and numeral III represents the curve of the coercive forceHe, respectively.

FIG. 7 is a graph illustrating the relationship between the initialpermeability 1.00, the magneticflux density B, and the-coercive force Heof aternary alloy (FeNi), Mo, and where the amount 1: of Mo containedtherein is changed and the alloy FeNi is also selected to be Fe,,,,. Nias in the compositions of FIG. '6. In the graph of FIG. 7, curve I showsthe initial permeability 1.00, curve II the magnetic flux density B andcurve III the coercive force He, respectively.

In general, material for a magnetic shield case, especially for amagnetic shield case of a magnetic head used for recording only, isrequired to have initial permeability uo, greater than 3,000 andmagnetic flux density B greater than 3,000 Gausses. Further, the coercive force Hc, must be selected smaller than 0.] Cersted. Accordingly,in order to make the coercive force He smaller than 0.1, the amount x ofCr added to the alloy must be selected greater than about 6 weightpercent, as is apparent from FIG. 6, which shows compositions of theternary alloy containing no M0. The fact that the addition of an amountx of Cr must be selected more than 6 weight percent may also beunderstood from, for example, FIG. 8 which shows the composition of aquaternary alloy (FeNi)CrMo and the rust forming characteristic thereof.That is to say, FIG. 8 illustrates the anti-corrosive chracteristics ofthe quaternary alloy (FeNi)CrMo, in which the amounts of Cr and Mocontained therein are varied, respectively. In FIG. 8, values describedin the vicinity of the circles represent an incremental ratio of weightper unit time when a sample is oxidized at temperatures of 600 C., whilethe values in parentheses designate the decremental weight of a samplewhich is immersed in salt water of 8 percent for 100 hours. From FIG. 8it will be observed that if the additional amount of Cr is smaller than6 weight percent with respect to the total weight of the quaternaryalloy, the rust forming rate K becomes greater than bility uo, magneticflux density B and coercive force Hc thereof when the amounts of C r andM0 in the quaternary alloy (FeNi)CrMo are changed. The upper, middle andlower numerals at marked points on this diagram represent the values ofcoercive force, magnetic flux density, and initial permeability,respectively, of alloys having compositions corresponding to suchpoints. Since the initial permeability uo, of material used for a shieldcase must be greater'than 3,000 as mentioned above, it will be seen fromFIG. 6 that the amount of Cr must be selected lower than about 12 weightpercent with respect to the total amount of the quaternary alloy, whilethe amount of Mo must be selected higher than about 0.5 weight percentwith respect to the total amount of the quaternary alloy. In thisdiscussion, the amounts of Fe and Ni are assumed substantially equal inweight ratio. It is, however, observed similarly from FIG. 10, in whichthe weight ratio of Fe to Ni is made different, that if the amount of M0is selected lower than 0.5 weight percent relative to the total amountof the quaternary alloy, its initial permeability uo, is decreased.

FIG. 10 is a graph showing the magnetic characteristic of a quaternaryalloy designated by (Fe Ni,,,)- Cr Mo,, that is to say, the initialpermeability uo, magnetic flux density B and coercive force Hc thereof,when the amount x of M0 is changed in weight percent. From FIG. 10 itwill be observed that when the amount of M0 is selected smaller than 0.5weight percent, relative to the total amount of the quaternary alloy,its initial permeability uo, is greatly increased. Similarly, when theamount of M0 is selected higher than 8 weight percent, its magnetic fluxdensity B becomes lower than 3,000 (which is not shown but wasascertained by experiment). Further, the quaternary alloy thus obtainedbecomes uneconomical. Accordingly, in preferred embodiments of themagnetic alloy of the invention, the amount of M0 is selected lower thanabout 8 weight percent and greater than about 0.5 weight percent.

Based upon the above results, the magnetic material or alloy of thepresent invention which mainly consists of (FeNi)CrMo is formulated tohave a ratio of Ni to Fe (Ni/Fe) ranging from about 0.54 to about 1.5weight percent, about 6 to about 12 weight percent of Cr and about 0.5to about 8 weight percent of Mo. Such alloys are formed by conventionalalloy techniques which include heat-treatment in a magnetic field andneed no further explanation.

The following Table shows, by way of example, various characteristics ofthe conventional 79-Permalloy, 45-Permalloy and the magnetic materialaccording to the present invention.

'IKELE "W 45-Per- 79-Per- (Fe .,,,Ni,-,,,),, Cr,,Mo (Fe,, NiCr,,Momalloy malloy B[Gau 3] l5000 7000 9070 5500 #0 3000 10000 1580018000 HclOe] 0.l 0.0l 0.049 0.03 KlmglCmhr] 0.07 0.006 0.005 0.0]

0.01 and tends to lower the anticorrosive characteristic of the alloy.In this discussion, it is assumed that Fe and Ni are substantially equalin weight.

FIG. 9 is a diagram showing the composition of a quaternary alloy(FeNi)Cr Mo and the initial permea- From the above Table, it will beobserved that the magnetic material of the present invention hasmagnetic characteristics B, uo, Hc, equal to or higher than that of the45-Permalloy and a rust forming rate K, substantially equal to that ofthe 79-Permalloy. Further, it

will be also understood that the magnetic material according to thepresent invention can be economically manufactured and economicallyworked or processed.

FIG. ll shows a magnetic head I which is used for the wear test of themagnetic alloys according to the present invention. The magnetic head 1consists of a shield case 2 provided with two windows 3 and two magnetichead cores 4 housed in the shield case 2. In this case, the magnetichead cores 4 are molded in the shield case 2 with a resin and both ofthe magnetic head cores 4 and the shield case 2 have the same tapecontact surface.

FIGS. 12A to 12D, inclusive, are graphs showing worn states of the tapecontact surface of magnetic heads after tape running tests, in which thematerials for the shield case 2 and the magnetic head cores 4 of themagnetic head 1 shown in FIG. 1] were varied. In this test an ordinarymagnetic tape was caused to travel in contact with the contact surfaceof the respective magnetic heads for 200 hours at a predeterminedpressure. In the Figures, the dotted line shows the level of the contactsurface of the magnetic head before the IESI.

FIG. 12A corresponds to the situation where both of the shield case 2and the magnetic head cores 4 were both composed of 79 Permalloy. FromFIG. 12A it will be apparent that the shield case 2 and the magnetichead cores 4 were substantially uniformly worn to a relatively largedegree.

FIG. 12B corresponds to the situation where the shield case 2 was madeof 79-Permalloy while the magnetic head cores 4 were made of hardPermalloy. In this situation, the magnetic head cores 4 were worn lessbut the shield case 2 was worn by similar amounts as that shown in FIG.12A.

FIGS. 12C and 12D correspond to the situation where the shield case 2was made of the magnetic alloys according to the present invention, inthis example, the quaternary alloy (FeNi)Cr Mo while the magnetic headcores 4 were made of the 79-Permalloy and hard Permalloy respectively.It will be apparent from FIGS. 12C and 12D that the shield case 2 madeof the magnetic alloys according to the present invention is not onlyless worn but acts to protect the magnetic head cores from being worn.In other words, the magnetic alloys of the present invention are mostpreferable when used in combination with a magnetic head.

It may be possible to add one or more ofthe materials selected from thegroup consisting of niobium (Nb), titanium (Ti) and vanadium (V) to thequaternary alloys (FeNi)CrMo in a relatively small amount, for example,about 1 weight percent relative to the total amount of the alloy toenhance its hardness. In this situation, however, it should be notedthat if the amount of such further additive exceeds l weight percent thepermeability of thus obtained alloys becomes low and working thereofbecomes rather difficult.

In the above discussion, the magnetic alloys according to the presentinvention were used as a magnetic shield case for a magnetic head,however, the alloys of the invention are also useful as a magneticshield plate disposed between magnetic head cores.

It may also be possible that the magnetic alloys of the presentinvention can be used for ordinary magnetic shields.

It will be apparent that many modifications and variations may beeffected without departing from the scope of the novel concepts of thepresent invention.

I claim as my invention:

1. A magnetically heat-treated alloy consisting essentially of iron,nickel, about 6 to about 12 weight percent chromium, and about 0.5 toabout 8 weight percent molybdenum wherein a weight ratio of nickel toiron is in the range of about 0.54 to about 1.5, said alloy beingcharacterized as having an initial permeability greater than 3.000, amagnetic flux density greater than 3,000 Gausses, a coercive force lessthan 0.1 and a rustforming rate less than about 0.0l.

2. A magnetically heat-treated alloy according to claim 1 and includingless than about 1 weight percent of a material selected from the groupconsisting of niobium, titanium, vanadium and mixtures thereof.

3. A magnetically heat-treated alloy according to claim 1 whereinsaidalloy consists of (Fe Ni Q CrgMOz.

4. A magnetically heat-treated alloy according to claim 1 wherein saidalloy consists of (Fe Ni, ,,Cr- "MO2-

1. A MAGNETICALLY HEAT-TREATED ALLOY CONSISTING ESSENTIALLY OF IRON, NICKEL, ABOUT 6 TO ABOUT 12 WEIGHT PERCENT CHROMIUM, AND ABOUT 0.5 TO ABOUT 8 WEIGHT PERCENT MOLYBDENUM WHEREIN A WEIGHT RATIO OF NICKEL TO IRON IS IN THE RANGE OF ABOUT 0.54 TO ABOUT 1.5, SAID ALLOY BEING CHARACTERIZED AS HAVING AN INITIAL PERMEABILITY GREATER THAN 3,000, A MAGNETIC FLUX DENSITY GREATER THAN 3,000 GAUSSES, A COERCIVE FORCE LESS THAN 0.1 AND A RUST-FORMING RATE LESS THAN ABOUT 0.01
 2. A magnetically heat-treated alloy according to claim 1 and including less than about 1 weight percent of a material selected from the group consisting of niobium, titanium, vanadium and mixtures thereof.
 3. A magnetically heat-treated alloy according to claim 1 wherein said alloy consists of (Fe50Ni50)90Cr8Mo2.
 4. A magnetically heat-treated alloy according to claim 1 wherein said alloy consists of (Fe60Ni40)90Cr8Mo2. 